A) Problems in General Temperature Measurement of Liquids
A method for readily measuring the temperature of liquids and slurries at positions other than at the surface of the liquid has been elusive. An example will serve to illustrate this problem.
Consider the difficulties in measuring the water temperature at the bottom of a lake fifty feet deep. Since thermometers, such as those based on liquid in glass, liquid crystals or bimetallic strips rapidly respond to temperature changes, they cannot be used to make this measurement. When these devices are lowered to the bottom of the lake and then raised to the surface for reading they quickly lose the temperature reading of the water at the bottom of the lake. The temperature recorded by these devices may reflect that of the intervening water or even of the air above the lake. In any event the recorded value is unreliable and does not reflect the temperature at the bottom of the lake.
The usual method for making such a temperature measurement is to lower a thermocouple to the bottom of the lake and to read the temperature using a suitable meter and power supply. This method is cumbersome and does not lend itself to ready and repeated use.
Besides being expensive, the equipment has the usual drawbacks associated with electrical contacts, long lengths of wire and useful life of electronic equipment at high humidity conditions. In addition, many of these environments can be very demanding in terms of physical stress (e.g. fishing boats) and corrosiveness (e.g. brines), particularly in windy conditions.
Another problem area of temperature measurement is with slurrries where the suspended solids block the view of the temperature scale on the thermometer. Vats of paint, sewage, lakes of black brine, and rivers loaded with silt are some examples of these types of situations.
In these instances it is desirable to remove the thermometer from the liquid for reading. Cleaning of the thermometer may even be necessary. Normal thermometers lose their readings in just a few seconds on removal from such slurries.
Another problem area of temperature measurement is the area of corrosive and/or fuming liquids. Thermocouples are readily attacked by many corrosive liquids and strong fumes, such as acids, ammonia and caustic. Intensely smelling or volatile poisonous chemicals often discourage or prevent a person from making a close approach for a temperature measurement.